For example, in JP-A-8-121180, a throttle valve for adjusting an amount of intake air is disposed in an intake pipe on the upstream side from an intake manifold of each cylinder of an internal combustion engine. Further, a swirl control valve is disposed in a passage on one intake port side of two intake ports provided in each cylinder, while a fuel injection valve is disposed in a passage on the other intake port side. During a low or medium load operation in which the amount of intake air is small, the swirl control valve is closed to allow the intake air to flow only through the intake port on the fuel injection valve side, so that a swirl flow is generated to promote homogenization of an air-fuel mixture. During a high load operation in which the amount of intake air is large, the swirl control valve is opened to allow the intake air to flow into both intake ports.
In such a system, when switching operations between opening and closing of the swirl control valves of the respective cylinders are performed at the same time in a transient operation for switching between opening and closing of the swirl control valve, the amount of intake air may greatly change to cause torque shock. For this reason, timings of opening and closing of the swirl control valves of the respective cylinders are controlled to be shifted from each other, thereby preventing the rapid change of the intake air amount due to the opening and closing of the swirl control valve.
However, when the swirl control valve is opened to allow the intake air to flow into both intake ports during the high load operation, fuel may be injected only into one of the two intake ports, that is, an intake port on the side where the fuel injection valve is disposed. In this case, the air-fuel mixture cannot be sufficiently homogeneous.
In order to prevent the drastic change (torque shock) in amount of the intake air due to the opening and closing of the swirl control valve, it is necessary to control the swirl control valves of the respective cylinders such that the opening and closing timings of these valves are shifted from each other. This leads to complicated control of the swirl control valves, resulting in increased computation load of a control computer.
Furthermore, because a throttle valve for adjusting the amount of intake air and a swirl control valve for generating the swirl flow are required to be provided independently, the valve structure becomes complicated, resulting in an increase in cost.